Scrubbing a gas stream containing so&#39; &#39; and o&#39; &#39; with an absorbent containing a reducing agent

ABSTRACT

An input gas stream containing SO2 and O2 is continuously treated in order to remove a substantial portion of the SO2 therefrom by the steps of: (a) scrubbing the input gas stream with an aqueous absorbent containing an alkaline reagent and a reducing agent at conditions selected to result in a rich absorbent stream containing a water-soluble thiosulfate compound and a treated gas stream; (b) reacting at least a portion of the resulting rich absorbent stream with carbon monoxide at reduction conditions selected to produce the corresponding sulfide compound; (c) stripping hydrogen sulfide from at least a portion of the resulting sulfide-rich solution to form a regeneratred aqueous absorbent stream; and, thereafter, (d) passing at least a portion of the regenerated absorbent stream to the SO2-scrubbing step. The principal utility of this scrubbing process is associated with the problem of continuously removing a sulfur dioxide contaminant from flue or stack gas streams containing SO2 and O2 such as are typically produced in modern electrical power generating stations in order to abate a serious pollution problem and to enable the safe, nonpolluting burning of high sulfur fuels. Key features of this process are: the selective conversion of the absorbed SO2 to the corresponding thiosulfate compound in the scrubbing step, the subsequent reduction of the thiosulfate compound to the corresponding sulfide compound in a highly economic and selective manner, the minimization of undesired sulfate by-products during both the scrubbing and reduction steps and the use of a relatively cheap, continuously regenerated absorbent which has a high capacity for SO2 as well as a high efficiency for SO2 removal.

United States Patent E191 Urban Jan. 7, 1975 [54] SCRUBBINGA GAs STREAM CONTAINING 24M) 92 .WITH .A N .A BS0RBENT CONTAINING A REDUCING AGENT Peter Urban, Northbrook, Ill.

[73] Assignee: Universal Oil Products Company, Des Plaines, Ill.

[22] Filed: Mar. 2, 1973 [21] Appl. No.: 337,605

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 226,018, Feb. 14, 1972, which is a continuation-in-part of Ser. No. 9,968, Feb. 9, 1970, Pat. No. 3,644,087.

[75] Inventor:

Primary Examiner-G. O. Peters Attorney, Agent, or Firm-James R. Hoatson, Jr.; Thomas K. McBride; William H. Page, II

[5 7] ABSTRACT An input gas stream containing S0 and O is continuously treated in order to remove a substantial portion of the S0 therefrom by the steps of: (a) scrubbing the input gas stream with an aqueous absorbent containing an alkaline reagent and a reducing agent at conditions selected to result in a rich absorbent stream containing a water-soluble thiosulfate compound and a treated gas stream; (b) reacting at least a portion of the resulting rich absorbent stream with carbon monoxide at reduction conditions selected to produce the corresponding sulfide compound; (c) stripping hydrogen sulfide from at least a portion of the resulting sulfide-rich solution to form a regeneratred aqueous absorbent stream; and, thereafter, (d) passing at least a portion of the regenerated absorbent stream to the SO -scrubbing step. The principal utility of this scrubbing process is associated with the problem of continuously removing a sulfur dioxide contaminant from flue or stack gas streams containing S0 and 0 such as are typically produced in modern electrical power generating stations in order to abate a serious pollution problem and to enable the safe, nonpolluting burning of high sulfur fuels. Key features of this process are: the selective conversion of the absorbed S0 to the corresponding thiosulfate compound in the scrubbing step, the subsequent reduction of the thiosulfate compound to the corresponding sulfide compound in a highly economic and selective manner, the

minimization of undesired sulfate by-products during both the scrubbing and reduction steps and the use of a relatively cheap, continuously regenerated absorbent which has a high capacity for S0 as well as a high efficiency for S0 removal.

16 Claims, 1 Drawing Figure CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of my prior, copending application Ser. No. 226,018 filed Feb. 14, 1972 which in turn is a continuation-in-part of my prior application Ser. No. 9,968 filed Feb. 9, 1970 and issued as US. Pat. No. 3,644,087 on Feb. 22, 1972.

The subject of the present invention is a novel, continuous process for the selective removal of S from a gas stream containing S0 and O More precisely,

the present invention involves a novel SO -scrubbing step operated with an aqueous absorbent containing an alkaline reagent and a thiosulfate-forming reducing agent, coupled with a unique regeneration procedure which comprises: a primary reduction step wherein at least a portion of the resulting thiosulfate-rich absorbent from the scrubbing step is reacted with carbon monoxide at conditions selected to produce a sulfiderich aqueous effluent stream and stripping step wherein the absorbent solution is regenerated with liberation of hydrogen sulfide. In one important aspect, the present invention has to do with a continuous; closed-loop SO scrubbing process to which is charged a gas stream containing S0 and O and which is operated with highly efficient aqueous absorbent containing sodium hydroxide, sulfite or carbonate and a reducing agent selected from the group consisting of finely divided sulfur, a

polysulfide compound, a water-soluble sulfide compound and mixtures thereof, wherein the rich scrubbing solution withdrawn from the scrubbing step previously contains sodium thiosulfate, wherein the primary reduction step involves a selective reaction of carbon monoxide with sodium thiosulfate to yield sodium sulfide and/or hydrosulfide, wherein H 8 is stripped from the resulting solution with CO to form a regenerated absorbent stream, and wherein the production of undesired sodium sulfate by-product is controlled and minimized in each step of the process.

A major problem encountered in many areas of industry today is associated with the production of waste gas streams containing sulfur dioxideand oxygen. The problem essentially involves the disposal of these waste gas streams without causing substantial air pollution. This problem is an extremely complex one because of the wide variety of industrial sources that emit these sulfur dioxide-containing gas streams. One of the more common sources is associated with the combustion of sulfur-containing fuels in boilers, internal combustion engines, heating units, etc., to produce flue or stack gas streams containing significant amounts of sulfur dioxide. Similarly, waste gas streams of this type are generally produced by other industrial processes such as the smelting of sulfur-bearing ores, the refining of coke, the production of sulfur in a Claus process, the production of sulfuric acid, the sulfonation of organic material, the production of sulfur-containing fertilizer, the production of paper via a wood pulping process and the like industrial processes. It is well known that the indiscriminate discharge of these gas streams into the atmosphere results in a substantial air pollution problem because the sulfur dioxide has extremely detrimental effects on animal and plant life. In addition, the discharge of these gas streams into the atmosphere constitutes a waste of a valuable material because the sulfur contained in same is an industrial commodity. Many processes have been proposed for removal of sulfur dioxide from these gas streams. A large percentage of these proposed removal procedures involve contacting the sulfur dioxide containing gas stream with an aqueous absorbent stream which typically contain materials which chemically or physically react with the sulfur dioxide in order to absorb same into the liquid solution. A widely studied procedure involves the use of a solution of an alkaline reagent such as ammonium hydroxide, sulfite, or carbonate, one of the alkali or alkaline earth metal salts such as sodium hydroxide, sulfite or carbonate, potassium hydroxide, sulfite or carbonate and the like alkaline reagents, to produce a rich absorbent stream containing the corresponding sulfite and- /or bisulfite compound. For example, the use of an aqueous absorbent containing sodium bicarbonate and carbonate to form a rich scrubbing solution containing sodium sulfite and bisulfite.

Although the simple concept of the scrubbing S0 from the gas stream containing same with an aqueous absorbent containing an alkaline reagent has many advantages associated with it such as simplicity, high ef fectiveness and versatility, wide spread adoption of this type of solution to the S0 pollution problem has been inhibited by the lack of a regeneration procedure for the rich absorbent stream which can continuously regenerate the rich absorbent stream by selectively and economically converting the absorbed $0 to a conveniently handled and saleable product in a highly selective manner. That is, it is required that the regeneration procedure enable the operation of the scrubbing pro cess in a closed-loop system with respect to the absorbent. In particular, it is required that an acceptable regeneration procedure have the capability of not only continuously producing a regenerated absorbent stream but also minimizing undesired by-products so as to prevent the buildup of undesired intractable, difficulty removed ingredients in the absorbent stream once the system is operated in a closed-loop fashion. The byproduct that is of the greatest concern in this regard is sulfate for example, in a system using an aqueous solution of ammonium hydroxide or ammonium carbonate as the absorbent, ammonium sulfate and bisulfate salts once formed in the system can present serious problems if special means are not provided to remove it or its production is not suppressed or controlled. Specifically, these salts can build up until finely divided solids are formed which then can cause corrosion, erosion and fouling problems.

One solution that has been proposed to the problem of regenerating the rich absorbent streams of the types discussed above is the use of the suitable reducing agent to react with the sulfite compounds contained therein in order to selectively produce elemental sulfur and/or the corresponding sulfide compound. However, despite stringent precautions, when common reducing agents such as hydrogen, a suitable sulfide compound, or carbon monoxide are used in an attempt to directly reduce these sulfite compounds to elemental sulfur or the corresponding sulfide compounds, undesired sulfate compounds are formed in unacceptable amounts. These sulfate compounds are believed to be caused by the sulfite compounds undergoing auto-oxidationreduction at the conditions necessary for direct reduction.

Even if sulfate by-product formation can be controlled or suppressed while regenerating the rich absorbent stream from the SO -scrubbingstep, it does not necessarily insure that the problem of unchecked sulfate by-product formation has been eliminated from the scrubbing system. As was pointed out in my prior applications, when the input gas stream to the scrubbing system contains both S and 0 the formation of substantial amounts of sulfate by-product in the S0 scrubbing step is inevitably observed when it operated with conventional aqueous absorbents of the type previously characterized. The formation of this sulfate byproduct in the scrubbing step is attributed to the interaction of the oxygen contained in the input gas with the water-soluble sulfite compound which is formed when the S0 is absorbed in the scrubbing solution. Hence in the common case where the gas stream to be treated contains both S0 and 0 there is a substantial need for an SO -scrubbing process that operates with a closed-loop of absorbent and has the capability ofsuppressing sulfate by-product formation not only in the regeneration section but also in the scrubbing section.

The problem addressed by the present invention is, therefore, to provide a flue gas scrubbing system for an input gas stream containing both S0 and 0 which is capable of continuous operation with a closed-loop absorbent circuit which can selectively produce an easily removed substance as the principal product of the regeneration section and which can minimize the amount of undesired sulfate by-products formed both in the scrubbing section and in the regeneration section.

I have found a combination process for continuously scrubbing S0 from a gas stream containing both 80, and 0 which utilizes a conventional aqueous absorbent stream in a closed-loop fashion and which comprises a novel wet scrubbing step coupled with a novel regeneration procedure which enables the recovery of hydrogen sulfide in high yield, minimizes undesired sulfate by-products from both the scrubbing and the regeneration sections and produces a regenerated absorbent stream which is of a relatively low total sulfur content ,and, consequently, possesses a high capacity for S0 removal. The concept of my invention is based on my findings that ('1) the sulfite compound formed in the scrubbing step by interaction between the SO 'in the gas stream and the alkaline reagent can be selectively converted at conventional scrubbing conditions to the corresponding thiosulfate compound via reaction between a reducing agent, selected from the group consisting of finely divided elemental sulfur, a watersoluble sulfide compound, a polysulfide compound and mixtures thereof, at a much faster rate than the sulfite compound will react with the O component of the gas stream to form the undesired sulfate by-product; (2) the thiosulfate compound once formed has a very small propensity to be oxidized by the 0,; component of the gas to sulfate relative to the propensity of sulfite; and (3) the resulting thiosulfate-rich absorbent stream can be reduced by carbon monoxide in a highly selective, economic and efficient manner to form a solution of the corresponding sulfide compound from which hydrogen sulfide can be easily stripped to form the regenerated absorbent. Thus the central point of the present process involves using an sO -scrubbing procedure to make a rich absorbent stream containing thiosulfate coupled with a regeneration procedure wherein thiosulfate is used as an intermediate in a multi-step operation designed to convert the S0 contained in the input gas stream to hydrogen sulfide, rather than attempt to form only sulfite in this scrubbing step and thereafter directly reduce the resulting sulfite compound to sulfide. This sulfite-to-thiosulfate-to-sulfide route provides an SO -scrubbing system which facilitates careful control of sulfate by-product formation during both the scrubbing and regeneration operation and enables the production of regenerated absorbent stream which can be directly recycled to the SO -scrubbing step, thereby allowing the system to be operated in the closed loop fashion with respect to the absorbent stream.

It is accordingly, an object of the present invention to provide a simple, effective, efficient, and selective procedure for treating a gas stream containing S0 and 0 which process can selectively produce hydrogen sulfide and operate with a continuous closed-loop circuit of absorbent between the scrubbing section and the regeneration section. Another object is to minimize the amount of undesired, intractable by-products formed in both the regeneration and scrubbing sections of such a procedure. Another object is to provide a regeneration procedure for an SO -scrubbing step that maximizes the sulfur differential across the regeneration procedure, thereby increasing the capacity and efficiency of the regenerated absorbent.

In brief summary, one embodiment of the present invention is'a process for treating an input gas stream containing S0 and O in order to continuously remove a substantial portion of the S0 therefrom. The first step is a scrubbing step wherein the input gas stream is contacted, in a suitable liquid-gas contacting zone, with an aqueous absorbent containing an alkaline reagent and a reducing agent, selected from the group consisting of finely divided sulfur, a polysulfide compound, a water-soluble sulfide compound and mixtures thereof, at scrubbing and thiosulfate production conditions selected to form a treated gas stream containing a reduced amount of S0 and an effluent stream containing a thiosulfate compound. Thereafter, at least a portion of the effluent stream from the scrubbing is reacted with carbon monoxide in the primary reduction step at reduction conditions selected to produce a sulfidecontaining aqueous effluent stream. The next step is a stripping step wherein hydrogen sulfide is stripped from at least a portion of the aqueous effluent stream from the primary reduction step to produce a hydrogen sulfide-containing overhead stream and regenerated aqueous absorbent stream. In the final step, at least a portion of the resulting regenerated aqueous absorbent stream is passed to. the scrubbing step, thereby providing a closed-loop flow circuit of absorbent.

In another embodiment, the invention is a process as outlined about in the first embodiment wherein the alkaline reagent utilized in the aqueous absorbent stream is selected from the group consisting of the salts of ammonia, the alkali metals and the alkaline earth metals which hydrolize in water to form a basic solution for example, the hydroxide, sulfites and carbonate salts. The term basic is used here to connote a solution capable of absorbing S0 by formation of sulfite or bisulfite salts.

In a more specific embodiment, the present invention is a process for treating a gas stream containing S0 and O in order to remove a substantial portion of the S0 therefrom. The first step in this embodiment involves a scrubbing step in which the input gas stream is contacted with an aqueous absorbent containing sodium sulfite, hydroxide, carbonate or bicarbonate and a reducing agent, selected from the group consisting of finely divided sulfur, a polysulfide, a water-soluble sulfide compound and mixtures thereof, at scrubbing and thiosulfate production conditions selected to result in a treated gas stream containing a substantially reduced amount of S and in an effluent stream containing sodium thiosulfate. The primary reduction step then involves reacting at least a portion of the effluent stream from the scrubbing step with carbon monoxide at reduction conditions selected to produce an aqueous effluent stream containing sulfide or hydrosulfide. At least a portion of the aqueous effluent stream from this primary reduction step is thereafter subjected to contact with a carbon dioxide-containing stripping gas at stripping conditions effective to produce a rcgenerated aqueous absorbent stream containing sodium bicarbonate or carbonate and an overhead gaseous stream containing hydrogen sulfide. At least a portion of the resulting regenerated absorbent stream is then in the final step, passed to the scrubbing step, thereby providing a closed-loop flow circuit of absorbent.

In another embodiment, the present invention is a process as described above in the last embodiment wherein the reducing agent utilized in the scrubbing step is hydrogen sulfide and wherein at least a portion of the overhead stream produced in the stripping step is passed to the scrubbing step in order to supply a portion of the hydrogen sulfide reactant utilized therein.

Yet another embodiment of the present invention involves a process as outlined above in the first embodiment wherein the primary reduction step is performed in the presence of a catalyst comprising a catalytically effective amount of a metallic component selected from the group consisting of the transition elements of Groups VI and VIII of the Periodic Table, and compounds thereof, combined with a porous carrier material.

Other objects and embodiments of the present invention are hereinafter disclosed in the following detailed discussion of the input streams, the preferred conditions, the preferred reactants, the output streams and mechanics associated with each of theessential and preferred steps of the present invention.

The starting point for the subject process is a scrub bing step wherein an input gas stream containing S0 and O is contacted in a suitable gas-liquid contacting means with an aqueous absorbent containing an alkaline reagent and a reducing agent selected from the group consisting of elemental sulfur, a polysulfide compound, a water-soluble sulfide compound and mixtures thereof. As previously explained, the input gas stream passed to this step is typically a flue or stack gas. For example, a typical stack gas stream containing about 1 to about 10% 0 about 5 to 15% or more CO about 3 to about or more H O, about 0.01 to about 1% or more S0 In many cases, the input gas stream will also contain carbon monoxide, oxide of nitrogen, entrained fly ash and the other well-known ingredients for flue gas streams. The amount of S0 contained in this input gas stream can vary over a wide range; namely from about 0.01 to about 1 mole or more, with a more typical amount being about 0.05 to about 0.5 mole Similarly, the amount of O, in this stream can vary over a wide range of about I to about 20 and typically about 5 to In many cases, this input gas stream is available at a relatively high temperature of about 200 to about 500 F. or more, and since it is preferred that the temperature of the input gas stream be at a relatively low level because this increases the capacity of the absorbent solution, it is often advantageous to cool the input gas stream by any suitable means such as by presaturating it with water under adibatic conditions.

The aqueous absorbent utilized in this scrubbing step is generally characterized as an aqueous solution of a suitable alkaline reagent which reacts with water to give a solution which will absorb S0 such as ammonium hydroxide, ammonium carbonate and bicarbonate, ammonium sulfite, the alkali metal hydroxides, sulfites the alkali metal carbonates and bicarbonates and the water-soluble alkaline earth metal hydroxide, sulfites, carbonates and bicarbonates, and the like alkaline reagents. Of the alkali metal reagents, sodium hydroxide, sodium carbonate, sodium sulfite, sodium bicarbonate, potassium hydroxide, potassium carbonate, potassium bicarbonate are particularly preferred. In most cases, excellent results are obtained when the alkaline reagent is ammonium or sodium hydroxide, sulfite carbonate or bicarbonate. It is to be noted that it is within the purview of the present invention to use a mixture of the alkaline reagents previously mentioned. Since it is also contemplated that the scrubbing step can be operated with the absorbent continuously cycling around the scrubbing means, it is possible that absorbent could accumulate substantial amounts of sulfite and bisulfite compounds. In this last case, only a small portion of the rich effluent stream from the scrubbing step would be sent to the regeneration section of the process, and the major portion of the rich absorbent would be commingled with the regenerated absorbent stream and recycled to the scrubbing step.

The amount of alkaline reagent contained in the scrubbing solution is subject to some choice depending upon the specificrequirements associated with the particular gas stream being treated; ordinarly, acceptable results are obtained when the alkaline reagent comprises about I to about 50 wt. of the absorbent solution, and more preferably to 1 to about 15 wt. Of course, absorbent solutions containing an amount of the alkaline reagent up to the solubility limit of the particular alkaline reagent selected at the conditions maintained in the scrubbing step can be utilized if desired. In the case where the absorbent is continuously cycled around the scrubbing step and only a drag stream drawn off for regeneration, the total amount of the alkaline reagent contained in the solution (i.e., fresh and spent) can reach rather high levels; for example, it can easily constitute 5 to about 50 wt. of the absorbent solutions. When the alkaline reagent is sodium sulfite, excellent results are obtained when the scrubbing solution contains about 5 to about 10 wt. thereof.

This scrubbing step can be carried out in a conventional scrubbing zone in any suitable manner including multiple stages. The scrubbing solution can be passed into the scrubbing zone in either upward or downward flow and the input gas stream can be simultaneously introduced into the scrubbing zone in concurrent or countercurrent flow relative to the scrubbing solution. A particularly preferred procedure involves downward flow of the scrubbing solution with countercurrent flow of the gas stream which is to be treated. The scrubbing zone is preferably a conventional gas-liquid contacting zone containing suitable means for effecting intimate contact between a descending liquid stream and an ascending gas stream. Suitable contacting means include bubble trays, baffles, and any of the various packing materials knownto those skilled in the art.'In this countercurrent mode of operation, a treated gas stream is withdrawn from the-upper region of the scrubbing zone and a rich absorbent solution is withdrawn from the lower region thereof. For the class of alkaline reagents of concern here, the rich absorbent solution will contain substantial amounts of a water-soluble thiosulfate compound such as ammonium thiosulfate, sodium thiosulfate and the like. As indicated previously, according to one mode of operation of the scrubbing step, only a drag stream from the rich absorbent withdrawn from the step is sent to the regeneration section of the process; the rest is cycled around the scrubbing step. The drag stream is ordinarily withdrawn at a rate at least sufficient to continuously remove the net sulfur taken up in the scrubbing step. According to the present invention, the absorbent stream used in this scrubbing step also contains, a special type of reducing agent. This reducing agent is selected from the group consisting of finely divided sulfur, a polysulfide compound, a water-soluble sulfide compound and mixtures thereof. In the first mode of operation of this step, finely divided sulfur is utilized as the reducing agent, and it is preferred that the sulfur'be present in particle size of about 10 to about 250 microns, with best results obtained with particles of about 25 to about 100 microns. Typically, it is a good practice to introduce the sulfur into this step via a water stream containing a slurry of finely divided sulfur in an amount of about I to about 75 wt. thereof, although many other suitable means for injecting finely divided solid particles can be utilized if desired. In this first mode of operation, it is preferred to also utilize a wetting agent in the reaction mixture in order to facilitate good contact of the elemental sulfur with the sulfite compound. Suitable wetting agents are:

the salts of alkyl aryl sulfonates such as the sodium salt of dodecylbenzene sulfonate; sulfonated fatty acid esters; C to C alkyl sulfates; C to C alkyl sulfonates; alkyl polyoxyethylene'alcohols; ethylene oxide condensation products of alkyl phenolsjquaternary ammonium salts such as octadecyldimethylbenzyl ammonium chloride and the like wetting agents. The wetting agent is preferably utilized in a relatively small amount corresponding to about 0.01 to about 1 wt. of the sulfite compound that is reacted. The amount of elemental sulfur utilized in this first mode of operation of the scrubbing step should be sufficient to convert at least 90% of the absorbed S0 to the corresponding thiosulfate compound; that is, an amount sufficient to supply. one atom of sulfur per molecule of S0 contained in the input gas stream, with the preferred amount corresponding to about one to about three atoms of sulfur per mole of absorbed 80;.

In a second mode of operation for this scrubbing step, the reducing agent is a polysulfide compound. Suitable polysulfide compounds include the ammonium, alkali metal, and alkaline earth polysulfides. Best results are ordinarily obtained with ammonium or sodium polysulfide. The polysulfide compound is ordinarily charged to this first step in the form of an aqueous solution containing about 1 to about 50 wt. of the polysulfide compound. It is to be noted that when the reducing agent is a polysulfide compound, no wetting agent is necessary in order to achieve good contact with the absorbed S0 The amount of the polysulfide compound charged to this step is preferably sufficient to provide at least of the stoichiometric amount necessary for the reaction between it and the absorbed S0 to produce the corresponding thiosulfate compound. In the typical case where the polysulfite compound contains four atoms of elemental sulfur and one atom of sulfide (e.g. (NI-[0 8 the stoichiometric amount is l/6 moles of polysulfide per mole of absorbed S0 with a preferred value being about A to about or more moles of polysulfide permole of absorbed S0 In a third mode of operation of the present scrubbing step, the reducing agent is a water-soluble sulfide compound. Suitable wat'er-soluble sulfide compounds are hydrogen sulfide, ammonium sulfide, ammonium hydrosulfide and the sulfides and hydrosulfide salts of the alkali and alkaline earth metals. Best results are ordinarily obtained in this mode of operation of this step when the sulfide-reactant is hydrogen sulfide or sodium or ammonium hydrosulfide. The amount of this sulfide reactant utilized in this'mode is at least sufficient to provide about 0.45 moles of sulfide compound per mole of absorbed S0 with best results obtained at a mole ratio corresponding to about 0.5 to about 1.5 or more. This sulfide reducing agent is preferably obtained from an associated with the present process. One available source is a portion of the effluent stream from the hereinafter described primary reduction step. Another good source is a portion of the H s-rich overhead stream from the subsequently discussed H S stripping step. Still another internal source is a portion of the .sulfide charged to the sulfide stripping step which can be carried back to the SO -scrubbing step by means of the regenerated absorbent stream if the stripping conditions are selected so that the required amount of sulfide is left in the bottom stream from the stripping step.

This reducing agent can be introduced into the scrubbing step in any known manner, either in admixture with the input gas stream or the absorbent stream or independently of both of these. The preferred procedure is to admix the reducing agent with the absorbent stream prior to its introduction into the scrubbing zone. Another acceptable procedure is to introduce the reducing agent at a point substantially below the point of introduction of the absorbent stream in order to prevent carryover of the reducing agent from the top of the scrubbing zone in the treated gas stream. Where the reducing agent is a portion of the H s-containing overhead stream from the stripping step, the preferred practice is to admix it with the input gas stream prior to introducing same into the scrubbing step.

This scrubbing step is generally conducted under SO -scrubbing and thiosulfate production conditions which are selected on the basis of the characteristics of the specific alkaline reagent utilized, the sulfur dioxide and oxygen content of the input gas stream, the portion of the sulfur dioxide that is to be removed in the scrubbing step, and the physical properties of the scrubbing zone. Ordinarily, the scrubbing step is preferably operated at a relatively low temperature of about 10 to C., and preferably about 25 to 75 C a relatively low pressure which typically approximates atmospheric, a volume ratio of input gas streams to scrubbing solution of about 25:1 to about 10,000: 1, and preferably about 100:1 to about 300:1; and a pH of about 4 to about 7 or more and preferably about 5.5 to about 7. When the input gas stream is a flue or stack gas stream, means must ordinarily be provided for cooling the input gas stream to a relatively low temperature of about 10 to 100 C. before it is introduced into the scrubbing step. Likewise, since the typical operation of the scrubbing step involves the handling of large volumes of gas containing only a relatively small amount of sulfur dioxide, it is preferred that the pressure drop through the scrubbing zone be held to a minimum so as to avoid the necessity of compressing large volumes of gas to overcome the pressure drop within the scrubbing zone, therefore, it is preferred to run the scrubbing zone gas-full instead of liquid-full.

Following the scrubbing step, an aqueous effluent stream containing relatively large amounts of a thiosulfate compound is withdrawn therefrom and at least a portion of it passed to the primary reduction step of the present process wherein the thiosulfate is reacted with carbon monoxide at reduction conditions selected to produce the corresponding sulfide compound.

The carbon monoxide stream for use herein may be obtained from any suitable source or may be prepared in any suitable manner. An acceptable carbon monoxide stream is obtained by the partial oxidation of organic materials, and particularly carbon at high temperature with oxygen, air or steam. Likewise, a carbon monoxide stream suitable for use herein can be prepared by the reduction of carbon dioxide by hydrogen, carbon or certain metals at high temperatures. For ex ample, a gas stream containing about 40% carbon monoxide is easily prepared by blowing steam through a bed of coal at an elevated temperature. Another suitable carbon monoxide-containing stream is obtained by simultaneously blowing air and steam through a bed of red hot coal to produce a gas stream containing about 30% carbon monoxide. In addition, blast furnace gases resulting from the reduction of iron oxide by red hot coke can be utilized to supply the necessary carbon monoxide stream if desired. Yet another source of a suitable carbon monoxide stream is a stream prepared by passing carbon dioxide and oxygen through charcoal or coke at a temperature greater than about l,000 C. in order to decompose theCO to C0. Regardless of the source of the carbon monoxide, it is preferably used herein in an amount sufficient to provide a mole ratio of carbon monoxide to thiosulfate compound of at least 4:1, with best results obtained at a mole ratio of about :1 to :1 or more. I have observed that the amount of sulfide formed increases with higher mole ratios of carbon monoxide to thiosulfate.

This primary reduction step can be carried out, if desired, without the use of a catalyst; however, in many cases, it is advantageous to use a catalyst for this reac tion. Based on my investigations I have determined that improved results are obtained in this step when the reaction zone contains materials such as particles of charcoal, and particles of activated carbon. Particularly good results are obtained with a catalyst comprising a catalytically effective amount of a metallic component selected from the group consisting of the transition metals of Groups VI and VIII such as chromium, molybdenum, tungsten, iron, cobalt, nickel, platinum, palladium, etc. From this, I have concluded that preferred catalysts for the desired reduction reaction comprise a combination of a Group VI or a Group VIII transition metal component with a suitable porous support such as alumina or activated carbon. Particularly preferred embodiments of the present step involve the use of catalysts in which the metallic component is present in the form of a metallic sulfide such as cobalt sulfide, or molybdenum sulfide, or tungsten sulfide combined with a carrier material. The preferred carrier materials are activated carbons such as those commerically available under the trade names of Norite, Nuchar, Darco and other similar products. In addition, other conventional natural or synthetic highly porous inorganic carrier materials may be used as the support for the metallic component such as alumina, silica, silica-alumina, etc. Best results are ordinarily obtained with a catalyst comprising cobalt or molybdenum or tungsten sulfide combined with relatively small particles of activated carbon. Excellent results have been obtained with 10 to 12 mesh activated carbon particles containing about 5 wt. of cobalt sulfide. In general, the amount of the metallic component utilized in the catalyst can be any amount which is catalytically effective and ordinarily should be sufficient to comprise about 0.1 to about 50% thereof, calculated on a metallic sulfide basis. These catalysts can be prepared according to any of the conventional procedures for combining a metallic component with 9 carrier material, with an impregnation procedure with a soluble, decomposable compound of the desired Group VI or VIII transition metal ordinarily giving best results.

This primary reduction step can be carried out in a conventional reaction zone in any suitable manner. The thiosulfate-containing effluent stream from the preliminary treatment step can be passed into the reaction zone in either upward, radial or downward flow and the carbon monoxide stream can be simultaneously introduced into the reaction zone in either countercurrent or concurrent flow relative to the thiosulfatecontaining effluent stream. In particular, a preferred embodiment of this step involves downward flow of the thiosulfate stream with counter-current flow of the carbon monoxide stream. It is preferred to utilize suitable means in the reaction zone for effecting intimate contact between a liquid stream and a gas stream. Suitable contacting means include bubble trays, baffles and any of the various packing materials known to those skilled in the art. In the preferred case where a catalyst is utilized in this second step, best results are ordinarily obtained when the catalyst is maintained within the reaction zone as a fixed bed of relatively small particles. These catalyst particles perform the dual functions of catalyzing the desired reaction and of promoting intimate contact between the gas and liquid streams. In the preferred countercurrent flow mode of operation for this step, a gas stream containing carbon dioxide, unreacted carbon monoxide and some hydrogen sulfide is typically withdrawn from the upper region of the reaction zone. Likewise, an aqueous effluent stream con taining the corresponding sulfide compound is withdrawn from the lower region of the reaction zone. For example, in the case where the input stream to this primary reduction step'contains sodium thiosulfate, this aqueous effluent stream will primarily contain sodium hydrosulfide with minor amounts of unreacted sodium thiosulfate, sodium carbonate and sodium bicarbonate.

The reduction conditions utilized in this primary reduction step are typically relatively more severe than those utilized in the preliminary treatment step and can be generally characterized as reduction conditions sufficient to effect conversion of thiosulfate to sulfide. The temperature is preferably selected from the range of about 100 to about 350 C., with best results obtained at a relatively high temperature of about 150 to about 250 C. 'It is an essential feature of the present invention that the step is conducted under liquid phase conditions; accordingly, the pressure employed must be sufficient to maintain at least a portion of the effluent stream from the scrubbing step in the liquid phase. Typically the pressure is selected from the, range of about 100 to about 3,000 psig., and preferably about 500 to about 1,000 psig., as a function of the reaction temperature in order to maintain the desired liquid phase condition. Particularly good results are obtained at a temperature of about 200 C., and a pressure of about 750 psig. It is preferred to use a liquid hourly space velocity (defined on the basis of the liquid volume charge rate of the effluent stream from the first step divided by the volume of the reaction zone utilized in this second step in the case where a catalyst is not utilized and by the volume of the catalyst bed in the case where a catalyst is used in this second step) selected from the range of about 0.25 to about 10 hr., with best results obtained at about 0.5 to about 3 hr.. Excellent results have been obtained in this step with a LHSV of 1 hr..

In the next step of the present invention the sulfidecontaining aqueous effluent stream recovered from the primary reduction step is subjected to an H 8- stripping step designed to liberate hydrogen sulfide therefrom. Although any suitable stripping gas can be utilized including steam, nitrogen, air and the like, carbon dioxide is particularly preferred, because it acts to decrease the pH of the solution and form thecorresponding carbonate and bicarbonate salt. For instance in the case where the effluent stream from the primary reductionstep contains sodium hydrosulfide, stripping with carbon dioxide liberates hydrogen sulfide and produces a bottom stream containing sodium carbonate and bicarbonate. In another mode of operation of this step, the effluent water stream from the primary reduction step can be subjected to conditions sufficient to thermally decompose the sulfide compound contained in this stream. For example, in a typical case where this water stream contains ammonium hydrosulfide, acceptable decomposition conditions are a temperature of about 100 to 200 C., and a pressure of about 1 to about 75 psig. Typically this decomposition mode of operation is contacted in a conventional distillation zone wherein upflowing vapors are generated by sup plying heat to the bottom of same by means such as a steam coil or conventional reboiler. Regardless of which mode of operation is employed in this stripping step, an overhead stream containing hydrogen sulfide will be produced. Likewise, a regenerated aqueous absorbent stream which is substantially reduced in total sulfur content will be recovered therefrom as a bottom stream.

This regenerated aqueous absorbent stream is substantially reduced in total sulfur content relative to the rich absorbent stream withdrawn from the scrubbing step and usually will contain less than l0% of the amount of sulfur contained in the rich absorbent stream. In the case where the carbon dioxide is utilized in the stripping step as the stripping medium, this treated water stream will contain substantial amounts of the carbonate or bicarbonate salt of the alkaline reagent originally present in the input absorbent stream for example, in the case where the alkaline, reagent is ammonia, the regenerated absorbent stream will contain ammonium carbonate and/or bicarbonate, and in .the case where the alkaline reagent is sodium hydroxide or carbonate the treated water stream will contain sodium carbonate and/or bicarbonate.

In accordance with the instant process, at least a portion of the regenerated absorbent stream is passed to the So -scrubbing step. In one embodiment of the instant process, a portion of the hydrogen sulfidecontaining overhead stream produced in the stripping step is passed to the SO -scrubbing step in order to supply at least a portion of the reducing agent used therein. The remaining portion of this hydrogen sulfidecontaining stream is then recovered as one of the product streams from the instant process. The hydrogen sulfide contained in this product stream can be converted to elemental sulfur by any suitable oxidation procedure such as a conventional Claus process or to sulfuric acid or used per se.

Having broadly characterized the essential steps comprising the present process, reference is now made to the attached drawing from a detailed explanation of a working example of a preferred flow scheme for the present invention. The attached drawing is merely intended as a general representation of the flow scheme involved with no intention to give details about heaters, pumps, valves, and the like equipment except where a knowledge of these devices is essential to an understanding of the present invention or would not be selfevident to those skilled in the relevant art.

Referring now to the attached drawing, a flue gas stream enters the system via line 1 and is passed into the lower region of a conventional liquid-gas scrubbing means, zoneS. In this zone it is passed in countercurrent flow to a descending stream of absorbent solution which enters the upper region of zone 5 via line 7. The input gas stream contains about 5% O 12% C0 6% H O, 76.8% N and 0.2% S0 Zone 5 is a conventional gas-liquid contacting zone fitted with convention means such as baffles, trays, packing material and the like, for effecting intimate contact between an ascending gas stream and a descending liquid stream. Zone 5 is run gas-full in order to minimize pressure drop therethrough.

Also introduced into zone 5 is a liquid stream comprising the aqueous absorbent solution. It enters zone 5 via line 7 and is made up of two separate streams, one of which is a regenerated absorbent stream obtained from the regeneration section of the present system and the second of which is a major portion of the rich absorbent stream withdrawn from the lower region of zone 5 via line 7. The alkaline reagent present in this regenerated absorbent stream is primarily sodium carbonate with a minor amount of sodium bicarbonate.

According to the present invention, a reducing agent is also charged to zone 5 in an amount sufficient to convert at least about of the absorbed S0 to sodium thiosulfate. ln the case illustrated in the drawing the reducing agent can be obtained from either the effluent stream from the primary reduction step, the H 8 -containing overhead stream from the H s-stripping step or the bottom stream from the lbs-stripping step. In the first mode of operation, the reducing agent is sodium hydrosulfide which is passed to zone 5 from zone 9 by means of lines 11, 12, 18 and 7 when valve 13 is in the open position and valve 17 is closed. In a second mode of operation for the embodiment shown in the drawing, the reducing agent is hydrogen sulfide which is obtained from the H S-containing overhead stream from zone 14 by means of lines 15, 16 and 1 when valve 17 is open and valve 13 is closed. In the third mode of operation for the illustrated process, the reducing agent is sodium hydrosulfide which is obtained from zone 14 by means of lines 18 and 7 when valves 13 and 17 are closed and stripping conditions used in zone 14 are selected to leave the-reqired quantity of sulfide in the bottom stream from zone 14. The preferred mode of operation of the illustrated process is this third mode.

During start-up of the illustrated process, line 2 is used to introduce the required inventory of a solution of sodium hydroxide or carbonate and sodium hydrosulfide until zones 9 and 14 are operating as hereinafter described. Thereafter, line 2 is used to introduce makeup alkaline reagent in the form of sodium hydroxide or carbonate as required to make up for alkaline reagent lost from the system when sodium sulfate byproduct is purged via line 19 or eliminated by separation as hereinafter described.

A sodium thiosulfate-rich absorbent stream is then withdrawn from the bottom of zone via line 7 and a major portion of this stream is continuously cycled around zone 5 in order to allow the concentration of thiosulfate salts in the absorbent stream to build to relatively high levels. This procedure increases the capacity of the absorbent and minimizes the amount of the absorbent stream that must be cycled through the regeneration section of the system. The absorbent stream introduced into scrubber 5 via line 7 will accordingly contain a substantial amount of thiosulfate salts and minor amounts of sulfite and sulfate salts along with the alkaline reagent. Ordinarily, zone 7 is operated by monitoring the pH of the absorbent stream at the inlet to zone 7 and controlling the amount of the rich absorbent stream diverted to the regeneration section of the system at the junction of line 7 and 8 in response to a decrease in pH level. The preferred pH range is about 4 to about 7 or more, with best results ordinarily obtained in the range of about 5 to 7. With the system operating so that the absorbent stream introduced into zone 5 via line 7 is maintained at a pH level within this range, the rich absorbentstream withdrawn continuously from zone 5 via line 7 can typically contain about 1 to about or more wt. sulfur principally as sodium thiosulfate. In addition, minor amounts of sodium sulfate are formed in zone 5. In the particular case shown in the drawing, the rich absorbent preferably contains about 5 to about 15 wt. sulfur as sodium thiosulfate and only about 0.5 to about 1.5 wt. sulfur as sodium sulfate. This corresponds toa conversion of about 90 to 95% or more of the absorbed S0,, to sodium thiosulfate, with only about 1 to 10% of the absorbed 50,, going to the undesired sodium sulfate byproduct.

Zone 5 is operated at a temperature of about 50 C., a pressure of about atmospheric and a gas to absorbent volume ratio of about 200:1. At these conditions, the treated gas stream withdrawn from the upper region of zone 5 via line 6 is found to contain less than 5% of the SO, originally present in the input gas stream.

As previously explained, the rich absorbent stream withdrawn from the lower region of zone 5 via line 7 is divided into two portions at the junction of line 7 with line 8. The major portion continues on via line 7 and is admixed with regenerated absorbent at the junction of line 18 with line 7 in an amount sufficient to provide about 0.7 moles of sodium carbonate per mole of S0 entering zone 5. The resulting mixture of cycled and regenerated absorbent is then reintroduced into zone 5 via line 7. The minor portion of the rich absorbent is passed via line 8 into the primary reduction zone, zone 9. The amount of rich absorbent passed into zone 9 is ordinarily at least sufficient to remove the net sulfur absorbed in zone 5 from the input gas stream in order to line out the concentration of sulfur in the absorbent stream. In the case under consideration, the amount of the absorbent withdrawn for regeneration via line 8 will be about 0.1 to 10% of the rich absorbent stream withdrawn from the bottom of zone 5 and typically about 1% of this rich absorbent stream. It is to be noted that during start up of scrubbing zone 5, the inventory of the scrubbing solution needed for initiating operation is introduced into the system via lines 2, l8 and 7. It is also to be recognized that there is a net water make in the regeneration of the absorbent, a major portion of which ordinarily is removed from the system in the treated gas stream withdrawn from the system via line 6.

Although not shown in the attached drawing, it is within the scope of the present invention to subject the rich absorbent stream withdrawn via line 8 to any known procedure for separating sodium sulfate from a aqueous solution of sodium sulfate and sodium thiosulfate such as by selective precipitation with lime or by fractional crystallization. The resulting sulfate-depleted stream would then be charged to zone 9. It is to be recognized that although the present invention does markedly suppress sulfate formation in scrubbing zone 5, it still occurs to some extent and an exit port for it must be provided if the process is to operate for a prolonged period of time. The alternative to elimination of net sulfate make by a separation operation performed on the sulfate-thiosulfate mixture in line 8 is to allow the sulfate concentration to build in the absorbent stream to the point where a small stream withdrawn from the system via line 19 or at any other appropriate point can remove the net sulfate make.

Zone 9, the primary reduction zone, is a liquid-gas reaction zone designed to effect intimate contact between an ascending gas stream and a descending liquid stream. The rich absorbent stream from zone 5 is introduced into the upper region of zone 9 by means of line 8. Likewise, a carbon monoxide-rich stream is introduced into the lower region of zone 9 by means of line 3. Zone 9 contains a fixed bed of a catalyst comprising 10 to 12 mesh particles of activated carbon having a cobalt sulfide component combined therewith in an amount sufficient to result in a catalyst containing about 5 wt. cobalt. The amount of carbon monoxide introduced into the system via line 11 corresponds to a carbon monoxide to sodium thiosulfate mole ratio of about 55:1. The reduction conditions maintained in zone 9 by conventional means are a temperature of about 200 C., a pressure of about 750 psig. and a liquid hourly space velocity of 1 hr..

A sulfide-containing aqueous effluent stream is then withdrawn from the lower region of zone 9 via line 11 and at least a portion of it passed to stripping zone 14. As previously explained, in one mode of operation of zone 5, the reducing agent for use therein is obtained by diverting a portion of the effluent stream from zone 9 via linesll, 12, 18 and 7 to zone with valve 13 open. An overhead gaseous stream is similarly withdrawn from the upper region of zone 9 via line and passed to the lower region to the stripping zone 14. An analysis of the stream flowing through line 11 indicates that 99% of the sodium thiosulfate charged to zone 9 is converted therein to sodium hydrosulfide. A similar analysis of the overhead gas stream indicates that it contains relatively large amounts of carbon dioxide with minor amounts of unreacted carbon monoxide, hydrogen sulfide and water. It is within the scope of the present invention to subject the overhead gas stream from zone 9 to any known procedure for the removal of H 8 therefrom prior to passage of same into zone 14. At the junction of line 10 with line 4 additional quantities of CO may be added to the system in order to increase the efficiency of the stripping operation in zone 14. In many cases the amount of CO contained in the overhead stream from zone 9 is sufficient for the stripping step and the addition of CO via line 4 is not necessary.

In stripping zone 14, the aqueous effluent stream from zone 9 is countercurrently contacted with an ascending gaseous stream which essentially comprises the overhead gaseous stream from zone'9 plus any material vaporized in the bottom of zone 14 plus any C0 added by line 4. Zone 14 is typically operated at a relatively low temperature and pressure as compared to zone 9. In fact excellent results are obtained at a temperature of about 125 C., a pressure of about 25 psig, and a mole ratio of entering j'CO to entering sulfide of about 1:1. Although satisfactory results can be obtained by stripping at relatively low temperatures under nonboiling conditions, it is, of course, advantageous to use conventional means such as a stream coil or reboiler to heat the liquid in the bottom portion of zone 14 in order to further generate up-flowing vapors which aid in the liberation of H 8. Typically, good results are achieved in zone 14 when about 10 to about 30% of water entering zone 14 by means of line 11 is taken overhead.

An overhead gaseous stream is then withdrawn from zone 14 via line 15 and passed to the junction of line l6'with line 15. The major portion of this overhead stream is then withdrawn from the system via line 15. The gas stream withdrawn via line 15 contains the net sulfide produce of the present process, and it can be charged to any suitable process for the recovery of sulfur or the manufacture of sulfuric acid if desired; for example, this stream could be passed to a conventional Claus unit for recovery of sulfur via an oxidation procedure. This overhead gaseous stream contains a relatively large amount of hydrogen sulfide, carbon dioxide and minor amounts of carbon monoxide, and water. As previously explained in one mode of operation of zone 5, another portion of this overhead stream can be passed via lines 16 and l to zone 5 when valve 17 is open in order to supply the hydrogen sulfide reactant necessary for the conversion of sulfite to thiosulfate in zone 5.

A stream of regenerated absorbent is withdrawn from the lower region of zone 14 via line 18 and passed back to zone 5 via line 18 and 7. At the junction of line 19 with line 18, a portion of the regenerated absorbent stream can be withdrawn from the system via line 19 in order to purge sodium sulfate as mentioned earlier and to remove any portion of the water produced in the system that is not removed via line 6 and 15. This regenerated absorbent stream primarily contains a mixture of sodium carbonate and bicarbonate with minor amounts of sodium hydrosulfide. The exact amount of sodium hydrosulfide in this stream depends on the efficiency of the stripping operation performed in zone 14 as previously explained. In addition, this stream can contain minor amounts of sodium sulfate. The total sulfur content of this regenerated absorbent stream is typically less than 10% of the total sulfur content of the rich absorbent stream withdrawn from the scrubbing section of the system via line 8. Moreover, the amount of undesired sodium sulfate formed in the scrubbing and regeneration section of the system is less than 10% of the S0 charged to the process via line 1. Thus the SO,- scrubbing process of the present invention enables the continuous scrubbing of S0 from a gas stream contain ing S0 and O entering the system via line 1 with continuous regeneration and recirculation of absorbent in a closed-loop manner. In addition, the amount of undesired sodium sulfate formed in the scrubbing and regeneration sections of the system is held to extremely low levels.

It is intended to cover by the following claims all changes and modifications of the above disclosure of the present invention that would be self-evident to a man of ordinary skill in the gas treating art.

I claim as my invention:

1. A process for treating an input gas stream containing S0 and O in order to continuously remove S0 therefrom, said process comprising the steps of:

a. contacting the input gas stream with an aqueous absorbent containing an alkaline reagent and a reducing agent, selected from the group consisting of finely divided sulfur, a polysulfide compound, a water-soluble sulfide compound and mixtures thereof, at scrubbing and thiosulfate production conditions selected to result in a treated gas stream containing a reduced amount of S0 and a effluent water stream containing a water-soluble thiosulfate compound;

b. reacting at least a portion of the effluent stream from step (a) with carbon monoxide at reduction conditions selected to produce a sulfide-containing aqueous effluent stream;

c. stripping hydrogen sulfide from at least a portion of the aqueous effluent stream from step (b) to produce a regenerated aqueous absorbent stream and on Pi s-containing overhead stream; and thereafter,

d. passing at least a portion of the resulting regenerated aqueous absorbent stream to step (a).

2. A process as defined in claim 1 wherein the alkaline reagent utilized in the absorbent is ammonium car- 7. A process as defined in claim 1 wherein the scrubbing and thiosulfate production conditions utilized in step (a) include a temperature of about 10 to about 100 C.

8. A process as defined in claim 1 wherein the reduction conditions utilized in step (b) include a temperature of about 100 to about 350 C. and a pressure sufficient to maintain the effluent stream from step (a) in the liquid phase.

9. A process as defined in claim 1 wherein the amount of carbon monoxide charged to step (b) is sufficient to provide a mole ratio of carbon monoxide to thiosulfate of at leat 4:1.

10. A process as defined in claim 1 wherein the reducing agent utilized in step (a) is obtained by passing a portion of the effluent stream from step (b) to step (a).

11. A process as defined in claim 1 wherein the reducing agent utilized in step (a) is obtained by controlling the stripping conditions used in step so that sufficient sulfide remains in the regenerated aqueous absorbent stream which is thereafter passed to step (a).

12. A process as defined in claim 1 wherein the amount of reducing agent charged to step (a) is sufficient to convert at least of the absorbed S0 to the corresponding thiosulfate compound.

13. A process as defined in claim 1 wherein step (b) is performed in the presence of a catalyst comprising a catalytically effective amount of a metallic component selected from the group consisting of the transition metals of Groups VI and VIII of the Periodic Table combined with a porous carrier material.

14. A process as defined in claim 1 wherein the catalyst used in step (b), comprises a catalytically effective amount of cobalt sulfide combined with activated carbon or alumina.

15. A process as defined in claim 1 wherein the stripping conditions utilized in step (c) include a temperature of about to about 200 C. and a pressure of about 1 to about 750 psig.

16. A process as defined in claim 1 wherein step (c) is performed with a stripping gas comprising carbon dioxide.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 859 416 Dated 3 Inventor (s) Peter Urban It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the cover-sheet following the designation of the "Assignee" insert the following:

[*1 Notice: The portion of the term of this patent subsequent to April 6, 1988, has been disclaimed.

Signed and sealed this 29th day of April 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks USCOMM-DC GONG-P69 i u.s. GOVERNMENT PRINTING OFFICE l9! o-ass-au.

F ORM PO-105O (10-69) 

1. A PROCESS FOR TREATING AN INPUT GAS STREAM CONTAINING SO2 AND O2 IN ORDER TO CONTINUOUSLY REMOVE SO2 THEREFROM, SAID PROCESS COMPRISING THE STEPS OF: A. CONTACTING THE INPUT GAS STREAM WITH AN AQUEOUS ABSORBENT CONTAINING AN ALKALINE REAGENT AND A REDUCING AGENT, SELECTED FROM THE GROUP CONSISTING OF FINELY DIVIDED SULFUR, A POLYSULFIDE COMPOUND, A WATER-SOLUBLE SULFIDE COMPOUND AND MIXTURES THEREOF, AT SCRUBBING AND THIOSULFATE PRODUCTION CONDITIONS SELECTED TO RESULT IN ATREATED GAS STREAM CONTANING A REDUCED AMOUNT OF SO2 AND A EFFLUENT WATER STREAM CONTAINING A WATER-SOLUBLE THIOSULFATE COMPOUND; B. REACTING AT LEAST A PORTION OF THE EFFLUENT STREAM FROM STEP (A) WITH CARBON MONOXIDE AT REDUCTION CONDITIONS SELECETED TO PRODUCE A SULFIDE-CONTAINING AQUEOUS EFFLUENT STREAM; C. STRIPPING HYDROGEN SULFIDE FROM AT LEAST A PORTION OF THE AQUEOUS EFFLUENT STREAM FROM STEP (B) TO PRODUCE A REGENERATED AQUEOUS ABSORBENT STREAM AND ON H2SCONTAINING OVERHEAD STREAM; AND THEREAFTER, D. PASSING AT LEAST A PORTION OF THE RESULTING REGENERATED AQUEOUS ABSORBENT STREAM TO STEP (A).
 2. A process as defined in claim 1 wherein the alkaline reagent utilized in the absorbent is ammonium carbonate, bicarbonate, hydroxide or sulfite.
 3. A process as defined in claim 1 wherein the alkaline reagent utilized in the absorbent is an alkali metal hydroxide, carbonate, bicarbonate or sulfite.
 4. A process as defined in claim 3 wherein the alkali metal is sodium.
 5. A process as defined in claim 3 wherein the alkali metal is potassium.
 6. A process as defined in claim 1 wherein the alkaline reagent utilized in the absorbent stream is an alkaline earth metal hydroxide, carbonate or sulfite.
 7. A process as defined in claim 1 wherein the scrubbing and thiosulfate production conditions utilized in step (a) include a temperature of about 10* to about 100* C.
 8. A process as defined in claim 1 wherein the reduction conditions utilized in step (b) include a temperature of about 100* to about 350* C. and a pressure sufficient to maintain the effluent stream from step (a) in the liquid phase.
 9. A process as defined in claim 1 wherein the amount of carbon monoxide charged to step (b) is sufficient to provide a mole ratio of carbon monoxide to thiosulfate of at leat 4:1.
 10. A process as defined in claim 1 wherein the reducing agent utilized in step (a) is obtained by passing a portion of the effluent stream from step (b) to step (a).
 11. A process as defined in claim 1 wherein the reducing agent utilized in step (a) is obtained by controlling the stripping conditions used in step (c) so that sufficient sulfide remains in the regenerated aqueous absorbent stream which is thereafter passed to step (a).
 12. A process as defined in claim 1 wherein the amount of reducing agent charged to step (a) is sufficient to convert at least 90% of the absorbed SO2 to the corresponding thiosulfate compound.
 13. A process as defined in claim 1 wherein step (b) is performed in the presence of a catalyst comprising a catalytically effective amount of a metallic component selected from the group consisting of the transition metals of Groups VI and VIII of the Periodic Table combined with a porous carrier material.
 14. A process as defined in claim 1 wherein the catalyst used in step (b), comprises a catalytically effective amount of cobalt sulfide combined with activated carbon or alumina.
 15. A process as defined in claim 1 wherein the stripping conditions utilized in step (c) include a temperature of about 100* to about 200* C. and a pressure of about 1 to about 750 psig.
 16. A process as defined in claim 1 wherein step (c) is performed with a stripping gas comprising carbon dioxide. 